1. Field of the Invention
The invention concerns the production of sacrificial casting cores for metal casting, in particular sacrificial casting cores of green or fired ceramic, which include metallic reinforcing elements, and their removal from the metallic castings, as well as principal molds for the production of casting cores.
2. Related Art of the Invention
The manufacture of cast parts with recesses or cutouts places high demands on the manufacturing techniques and the materials for the corresponding casting cores. In the field of metallic casting, due to the high temperatures ceramic molds are employed as a rule.
Slip casting is frequently used in the production of the ceramic casting cores, wherein shaping occurs by pouring liquid slip into a precursor or principal mold. Another frequently seen process is ceramic injection molding, wherein a formable ceramic mass is introduced under pressure into a precursor shape. The slip or ceramic mass is thereupon solidified by drying or, as the case may be, cooling, whereby a green ceramic shape is formed. Particularly in the case of complex shaped casting molds with fine, in part cantilevered or self-supporting structures, there are problems in removal from the mold and problems in the later metallic casting, which are attributable to the insufficient structural stability of the fired and, in particular, the green casting mold.
Already at the time of removal of the green ceramic out of the principal mold the insufficient stability of the material can lead to breakages of the fine structures. The removal of binder from the green ceramic results in general in a substantial mechanical weakening of the casting mold. In this way, in the case of improperly designed structural geometry, defects or breaking of the fine structure or self-supporting mold parts of the casting can occur.
A further source of defects during casting can be traced back essentially to the different densities of the ceramic and the casting metals used, in particular the iron alloys or steels. Since the ceramic in general has a substantially lower specific density than the casting metal, the fine and, in part freely projecting parts of the ceramic casting mold tend to float in the molten metal. This leads to geometric shape defects in certain areas of the cast.
The problem of the insufficient structural stability can in principle be addressed by increasing the sturdiness of the ceramic, for example by ceramic firing (sintering). This however has the serious disadvantage, that the casting mold can only be removed from the cast shape with substantial difficulty following casting. This is the case particularly in the case of casting hollow structures, where the remaining ceramic material is accessible with difficulty.
Further yet, the sintering of the ceramic generally leads to an unacceptable reduction in porosity.
Removal of a crust of remaining ceramic material out of the internal space of the castings is disclosed in JP 55097844 A1. This document discloses among other things polymer bound sand casting molds for casting of metal, reinforced with a spiral or helical shaped metal wire. The start and the end of the metal wire project out of the mold core. After the casting of the metal, the metal wire is pulled out of the casting whereupon the core of casting sand is broken up.